JPH1082887A - Method for removing dust from nuclear fuel pellet, and device therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To remove dust generated by grinding and accumulated on the surface of a nuclear fuel pellet, by laser beam impact. SOLUTION: The method for removing dust from at least one nuclear fuel pellet 7 subjected to grinding and having dust of material forming the nuclear fuel pellet, remaining on the pellet surface, has a process of exposing this surface to impact of laser beams 3 with an ideal characteristic to remove dust attachment, and a process for removing a product generated from the removing on dust attachment. In order to exposed the this surface to the impact of the laser beams 3, a motion mechanism 19 imparting translation to the laser beams 3 to the pellet 7 in combination with rotation of the pellet 7, a member 8 for recovering the product, a mirror 5 for leading the laser beams 3 to the pellet 7, and a laser beam protective screen 23 are arranged in a glove box 2 connected to a laser source 1 and provided with observation windows 21, 22.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing dust from nuclear fuel pellets by using a laser beam. The invention further relates to an apparatus for performing the method.

[0002]

BACKGROUND OF THE INVENTION Fuels used in the nuclear industry are:
Each pellet column is formed from elements having a cylindrical geometry constituted by pellet columns,
Housed in a metal container. The pellets can be made from uranium dioxide having a variable uranium 235 content, which can include other oxides such as plutonium dioxide (Pu
O ₂ ), or not. Pellets formed from a mixture of uranium dioxide (UO ₂ ) and plutonium dioxide (PuO ₂ ) are known as MOX (mixed oxide) pellets. These pellets
UO ₂ powder or UO ₂ powder and PuO, depending on the pellet type
It is produced by sintering at a high temperature after solidifying the mixture with the _two powders. These pellets can be selectively ground to respect diameter requirements. This grinding is essential if the manufacturing process cannot make it possible to directly adhere to the final diameter tolerance without performing the grinding. For example, in the case of pellets used in light water reactors.

[0003] Said grinding is achieved by machining the pellets between two grinding wheels, which generates dust, some of which are:
An outer shell is formed on the cylindrical surface of the pellet and remains. Grinding dust consists of micron or sub-micron units of particles composed of the same substance as the material of the pellet. The irradiation dose of dust varies depending on the properties of the grinding wheel used, the machining thickness, and the like.

[0004] The dust, which adheres more or less to a certain amount to the ground pellet surface, then deposits on the equipment and forms a particularly pronounced radiation source in the case of plutonium dioxide based fuels. Where americium forms in increasing amounts over time. Further, the dust interferes with the final automated sorting operation of the pellets, and
This may cause the pellets to be incorrectly rejected. The dust also causes dust to accumulate on the canning device when the pellets are stored in a metal container to form a fuel rod.

[0005]

SUMMARY OF THE INVENTION The present invention aims at obviating the disadvantages mentioned above. The present invention relates to nuclear fuel pellets, especially MOXs that are ground and arrive in a continuous flow manner.
Enables dust removal from pellets. As a result of the removal of grinding dust according to the invention, the following advantages are obtained:-radioactive material particles downstream of the grinding device (residual contamination).
Eliminating dust formation in equipment and equipment located in the glove box;-automatic appearance control is performed on the pellets, allowing correct sorting of the pellets not to be inadvertently made; Eliminates accidental rejection of some of the pellets;-Eliminates dust formation in the canning device and consequently reduces the fuel rods during the canning operation of the pellets to form fuel rods. Pollution is avoided.

[0006] Devices for removing dust from nuclear fuel pellets by laminar gas flow are already known and are disclosed in French patent publication A-2699317. The device comprises:
It has a dust removal tube in which the pellets circulate and move, whereby dust removal can be achieved. The tube has a gas inlet, a pellet introduction opening at its inlet end,
And means are provided for generating a laminar dedusting gas stream at a velocity of at least 25 m / s around the pellets circulating in the tube, said gas stream being parallel and being in the same direction as the pellet displacement direction. The dust removal tube is further provided with a dust / gas suction port and a dust removal pellet collection opening at an outlet end thereof.

The present invention proposes the use of a laser beam having a suitable power to remove dust from nuclear fuel pellets, and in particular dust that has formed its shell on the surface of the pellets during grinding.

It is already known to use the impact of a laser beam to clean the surface of an object. Thus, it is possible to clean surfaces contaminated by the deposition of compounds coming from the environment in order to restore the initial state, for example to restore works of art. Thus,
EP-A-380387 describes a method for cleaning the surface of a material (especially stone, glass, steel, ceramic, wood, paper, cardboard, etc.) using a laser beam which is focused on the surface to be cleaned. Has been disclosed. The "peak power" of the laser used for this purpose is set at a value between a few hundred kW and a few dozen MW. The diameter of the laser beam on the surface to be cleaned is a few M
The "peak power" density between W / cm ² and a few dozen MW / cm ² is selected to obtain on the surface. The disclosed apparatus has one laser and an apparatus for splitting the laser beam into another plurality of laser beams. The field of application of the procedures disclosed in the patent literature is in particular in the field of works of art (historical monuments, wooden furniture, glass, pottery) and cleaning or subsequent descaling during the production of tubing In the field.

[0009] It is also known to use laser beams to control the contamination of solid surfaces. For example, French Patent Publication A-2714464 discloses a laser contamination control method that employs a device for aspirating particles, particularly for the purpose of analyzing particles emitted by a laser beam. The device has an enclosure with an opening covering the surface area to be controlled. In the enclosure, to extract the particles to be sampled by the impact of the laser beam on the area,
A transparent plate or foil for transmitting the laser beam is provided. The enclosure further has an inlet and an outlet to allow circulation of a gas to transfer particles, and the enclosure has a transparent plate for the gas to clean the transparent plate. An additional inlet located as close as possible can be provided.

[0010] The impact of a laser beam can also be used to collect material samples for analytical purposes. The laser beam causes the vaporization of the sample, and the vaporized material is, for example, condensed on a support (see DE-A-196 313 15).
Or use an inert carrier gas (US Pat.
20414).

[0011]

SUMMARY OF THE INVENTION In the present invention, the impact of a laser beam is used to remove contaminant layers of different materials deposited on the surface of an object or to collect a sample of material for analytical purposes. Not used to Impact with a laser beam having the appropriate power is used to cause photon deposits of the machined dust present on the surface of the nuclear fuel pellet. The mechanical cutting dust has the same properties as the material of the pellet,
Before that, it constituted part of the material.

The present invention is specifically intended to remove from the surface of the pellet any constituent particles that may detach from the surface of the pellet during subsequent processing operations of the pellet as a result of embrittlement due to grinding. I do.

Therefore, the present invention provides a method for removing dust from at least one nuclear fuel pellet which has been subjected to a grinding process to leave dust of a material constituting the nuclear fuel pellet on the pellet surface, wherein dust adhering material is removed from the surface. A method for removing dust from the surface, the method comprising exposing the surface to the impact of a laser beam having properties suitable for producing, and removing products from the surface from the dust deposit removal. .

Advantageously, the impact of the laser beam on said surface produces a laser shock effect (ie, the specific power provided by the laser is less than 1).
00 MW / cm ² ).

[0015] If the laser beam is stationary, the pellet can be moved to expose the entire surface thereof to the impact of the laser beam such that dust removal is performed.

In this case, if the pellets are:
If it is a rotating geometric cylinder, and if the surface to be dusted, except at its bases, is the cylindrical surface of the pellet, the movement of the pellet relative to the laser beam will be the major axis of the pellet. There may be a translational movement according to the main axis of the pellet associated with a rotational movement around the line.

The movement of the pellet, or group of, for example, three pellets, is such that if the spot of the laser beam is of a suitable size to place the pellet or group of pellets exposed to the impact within an effective range. Then, it may be merely a rotary movement of the pellet or group of pellets about the main axis of the pellet or group of pellets without requiring any translational movement with respect to the laser beam.

According to the present invention, there is provided an apparatus for removing dust from at least one nuclear fuel pellet which has been subjected to a grinding process for leaving dust of a material forming a nuclear fuel pellet on the pellet surface, wherein dust adhering matter is removed from the surface. A dust removing device, comprising: means for exposing the surface to an impact of a laser beam having characteristics suitable for causing the dust to adhere to the surface; and means for removing, from the surface, a product resulting from the dust deposit removal.

When the laser beam is stationary, the device preferably has a mechanism for moving the pellet to expose the entire surface to the impact of the laser beam so that dust removal is effected. .

[0020] Such a mechanism may have means for rotating the pellet itself. Means may also be provided for selectively translating the pellet relative to the laser beam.

The present invention will now be described in greater detail in connection with some non-limiting embodiments, with reference to the accompanying drawings, in which:

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The dust removal apparatus shown in FIG. 1 has a laser source 1 docked to a glove box 2, which is required when processing MOX pellets. The laser beam 3 enters the glove box 2 after passing through a transparent glass window 4 arranged on the wall of the glove box 2. Next, laser beam 3
Is guided by one or more deflecting mirrors, such as a mirror 5 carried by a bracket 6, to the area where the pellets 7 are arranged. The laser beam 3 reaches the pellet 7 after having traversed a suction device mainly constituted by a member 8 for collecting the product resulting from dust deposit removal. Said member 8 is shown in greater detail in FIG.

FIG. 2 also shows the laser beam 3 and the pellet 7 as viewed along one of its bases. The member 8 has a substantially cylindrical main body 9 arranged so that the laser beam 3 passes through the center. The upper part of the body 9 is a transparent plate 1 through which the laser beam 3 can be transmitted.
It has been machined to form a seat 10 for receiving the same. The upper end of the main body 9 is tapped to screw the cylindrical plug 12 for holding the transparent plate 11 in place at the seat after the O-ring 13 is disposed in the middle.

The lower part of the body 9 is laterally perforated by a semi-cylindrical groove 14 having a diameter slightly larger than the diameter of the pellet 7. The groove 14 communicates with the lower opening 18 of the main body 9. Thus, the lower opening 18 is
It is shaped so as to overlap the pellet 7. The main body 9 has an opening 15 at a portion between the seat 10 of the transparent plate 11 and the groove 14. The opening is
It is located very close to the plate 11.

A duct 16 is connected to the lower part of the body 9 according to an axis inclined with respect to the laser beam 3. Said axis is directed towards the lower opening 18. The duct 16 is united with the main body 9 at an opening 17 called a suction opening.

As shown in FIG. 1, in this embodiment of the invention, three pellets 7 are arranged on a movement mechanism 19, which moves the pellets 7 in translation with respect to the laser beam 3. Thus, a movement combined with the rotation movement of the pellet 7 is generated. As a result of this double movement, the entire cylindrical surface of the pellet 7 is exposed to the impact of the laser beam. The unground pellet surface is not exposed to the laser beam impact. The movement mechanism 19 has a rod and locking means acting on the end of a column made of the pellet 7.

The dust removal device also has a supply group 20 for the laser, observation windows 21, 22 arranged on one of the walls of the glove box 2, and a laser beam protective screen 23.

A number of tests were carried out with the dust removal apparatus shown in FIG. 1 using a pulsed YAG laser operating in the infrared (for a wavelength of 1064 nm). The flash rate is a function of the speed of advance of the pellet under the laser beam. Satisfactory results have been obtained for flash rates of 5 Hz and 10 Hz. 20
Frequencies above Hz can be envisioned for industrial equipment. The pulse duration was about 7 ns. In this form, when a working power of 150 MW / cm ² ± 10 MW / cm ² was supplied to the pellet surface, the pulse per energy was 1850 mj. The laser beam spot is
Circular (diameter 7 mm, 9 mm or 11 mm), elliptical (dimensions, 19 mm × 10 mm) or rectangular (eg, 5 mm
0 mm x 2 mm). A rectangular spot is obtained by shaping the laser beam using an anamorphic cylindrical lens and a focusing lens. This rectangular spot
This is preferable for removing dust from pellets because it allows an optimal range for processing.

In industrial equipment, the pellets can be supplied continuously under a laser beam.

In order to improve the dust removal efficiency, it is preferable to suck the dust removed from the pellet.
This is a function of the suction member located as close as possible to the area of the pellet being processed. The opening 15 of the member 8 is connected to a neutral gas (for example, argon) or an air supply pipe (not shown). The duct 16
It is connected to a suction circuit (not shown).

For the tests performed, the suction speed was 2
It is preferably 0 m / s to 60 m / s, for example, 40 m / s. Place the opening 15 near the plate 11,
The gas curtain thus formed makes it possible to prevent dust removed from the pellets from accumulating on the plate 11. The effect of the gas curtain is enhanced by the lowering of the flow in the upper part of the body 9. This prevents the dust from being released and rising at a high speed by the photon deposit removal process. If the suction speed is too low, dust will accumulate in the duct 16.
If too high, excessive turbulence will occur. It is harmful for industrial use.

The first series of tests was carried out under the following conditions:-Number of pellet rotations: 50 times / min.-Pellet translation speed: 200 mm / min.-Laser power: 70 MW / cm ^{2 to} 160 MW / cm.
² ,-Number of laser passes: 1 or 2,-Laser frequency: 5 Hz or 10 Hz,-Suction flow speed: 50 m ³ / h

For frequencies of 5 Hz or 10 Hz, satisfactory results have been found to be obtained when the laser power is higher than 100 MW / cm ² in one or two passes.

A second series of tests was carried out under the following conditions: blown pellet base
bases), - the pellet rotation number: 97 times / min, - pellets translational speed: 1250 mm / min, - laser power: 40MW / cm ² ~160MW / cm
² , the number of laser passes: 1 or 2, the laser frequency: 10 Hz, the suction speed: 25 m ³ / h

A very satisfactory result is 130 MW / c
It has been found that for laser powers equal to or higher than m ² , especially when the base of the pellet is exposed to a blowing rate of 10 l / min.

A smear wipe test was also performed. Prior to laser treatment according to the invention, smears indicated the presence of dust, but after treatment no dust was found.

Surface roughness analysis performed on the samples obtained from the above tests showed that the surface roughness was not significantly corrected by the laser treatment.

The advantages of the method according to the invention are that the pellets do not deteriorate at all, that the dust removal rate is high and that the dust removal efficiency is excellent.

Inspection with a scanning electron microscope revealed that after the treatment according to the invention, no dust remained on the surface to be treated and that the pellets were not damaged at all.

The industrial dust remover shown in FIG. 3 makes it possible to automatically process the pellets coming out of the grinder. Said device is based on a kinematic chain formed from an automated belt and a vibrating feeder or bowl, which makes it possible to process n pellets simultaneously. In the embodiment shown in FIG.
It is.

The plurality of pellets to be subjected to the dust treatment accommodated in the vibration bowl 30 are supplied to the first electric belt 32 by the vibration rail 31. For example, the pellet advance speed is 15 mm on the vibrating rail 31.
/ Sec, and 30 m on the electric belt 32
m / sec. As a result of the oscillating belt 31, the plurality of pellets 33 are arranged on the electric belt 32 with their main axes parallel to the direction of advance, thus the pellets 33
Can be formed.

A second electric belt 34 is disposed on an extension of the first electric belt 32 for receiving the aligned pellets on the first electric belt 32. The connection between the electric belts 32 and 34 is constituted by a guide block or a sliding member 35. The second electric belt 34 may have a forward speed of 60 mm / sec.

The pellets 33 are continuously supplied to the second electric belt 34 as four columns by a holding device or a gripping device 36. Said gripping device makes it possible to hold the pellets on the guide block 35 or to release them. The column is formed by counting the pellets using an optical fiber device, with the optical fiber end 37 visible in the drawing.

A third electric belt 38 which partially overlaps above the second electric belt 34 and whose advancing speed is also 60 mm / sec, applies each column of the formed pellets to the dust removing area 40. And a plurality of tongues 39 arranged regularly over the surface of the belt for further advancement over the area and beyond.

The dust removal area 40 is located between two vertical panels 41 and 42. Panel 41 is the second
The flat part 43 for supplying the pellet column from the electric belt 34 to the dust removal area 40 is supported. The panel 42 supports a flat portion 44 for pulling the column from the dust removal area 40 to the pellet receiving tank 45. The panel 42 further supports a group 46, which controls the motor and the rotational movement caused by the motor, by two parallel rollers 47, 48 (only roller 47 is visible in FIG. 3). With a device for transmitting to The roller 47,
Reference numeral 48 is arranged between the panels 41 and 42 in a free rotation manner. The laser beam directed at the supported pellet column is shown in FIG. 3 by the reference numeral 50, which is rotated by the rollers 47, 48 and by the tongue 39 of the second electric belt 38. Can be propelled.

FIG. 4 is a cross-sectional view of rollers 47 and 48 for rotating the pellet 51 forming a part of the pellet column, which enable the processing of the pellet 51 by the laser beam 50. The rotation speed of the pellet may be one revolution per second. Further, a member 52 for recovering a product resulting from dust deposit removal is also seen. For reasons of clarity, said member 52 is not shown in FIG.

The member 52 includes a transparent portion 53 that allows the laser beam 50 to pass therethrough, ie, a transparent plate, and a pellet 5.
1 and has an opening 54 which allows the passage of the laser beam 50 and suction of the obtained product. As can be seen in FIG. 4, the laser beam 50 is inclined with respect to a vertical line to prevent dust from forming on the transparent plate 53 as a result of large particles or debris coming from the pellet being processed. Has been displaced.

The aperture 54 is elongated in the direction of the rollers 47, 48 to enable the pellet column to be processed over its entire length by a laser beam producing a 50 mm × 2 mm spot.

The distance between the rollers 47, 48 and the adjacent edge of the member 52 is made as small as possible. Roller 47,
As can be seen in FIG. 5, where 48 is shown longitudinally, the rollers 47, 48 are grooved to allow the introduction of carrier gas. When mounting the rollers 47, 48, they are preferably positioned in the manner shown in FIG. 5, ie, such that the grooveless portion 56 of the roller 48 corresponds to the groove 55 of the roller 47.

The member 52 has three suction nozzles, namely, a suction nozzle 57 provided at the lowermost portion of the member 52 and two horizontal suction nozzles 58, 59 adjacent to the opening 54.
Having. The suction nozzles 58 and 59 are connected to the suction duct 60,
61, each of which may have a diaphragm to balance the suction flow. The speed of the gas conveyed into these suction ducts is 20 m / s to 30 m / s
s. Lateral suction nozzles 58, 59 are provided with fish tail and
Advantageously, it is connected to said duct by a bit.

The slot 62 facing the inner surface of the transparent portion 53 allows for injecting gas with an adjustable flow rate to optimize dust removal for the transparent portion 53. This slot may be completed by another slot between the slot 62 and the lateral nozzle 59, which is located on the same slope as the member 52, thus creating a complementary air curtain on one hand and on the other hand It may help deflect the dust in the direction of the lower nozzle 57. Another configuration consists of facilitating suction by the lateral nozzles 58, which is disadvantageous for the lateral nozzles 59, so as to improve the deflection of the dust flow in the direction of the lower nozzles 57.

FIG. 4 also shows the oscillating mirror 65 and the laser beam shaping device 66. The oscillating mirror 65 enables periodic cleaning of the rotary rollers 47, 48 by the laser beam 50.

[Brief description of the drawings]

FIG. 1 is an overall view of a nuclear fuel pellet dust removing apparatus according to the present invention.

FIG. 2 is a view showing details of the nuclear fuel pellet dust removing apparatus of FIG. 1;

FIG. 3 is an overall view of an industrial nuclear fuel pellet dust removing apparatus according to the present invention.

FIG. 4 is a drawing of details of the apparatus of FIG.

FIG. 5 is a drawing of details of the apparatus shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser source 2 Glove box 3 Laser beam 4 Glass window 5 Mirror 6 Bracket 7 Pellet 8 Member 9 Main body 10 Seat 11 Plate 12 Plug 14 Groove 15 Opening 16 Duct 17 Opening 18 Lower opening 30 Vibration bowl 32 Electric belt 34 Electric belt 38 Electric belt

Claims (23)

[Claims] 1. A method for removing dust from at least one nuclear fuel pellet that has been subjected to a grinding process to leave dust of a material constituting the nuclear fuel pellet on the pellet surface, wherein the dust is removed from the surface. A method for removing dust, comprising: exposing the surface to the impact of a laser beam having suitable characteristics; and removing from the surface products resulting from removing the dust deposits. 2. The method according to claim 1, wherein the impact of the laser beam on the surface causes a laser shock effect. 3. The method of claim 1, wherein the pellet is moved while the laser beam is stationary while the pellet is exposed to laser beam impact such that the entire surface is subjected to dust removal. Method. 4. The method according to claim 3, wherein several pellets are arranged in one group, and the movement is performed by rotation of the group of pellets. 5. The method according to claim 3, wherein the pellet is a rotating geometric cylinder, the surface to be dusted is a cylindrical surface except at both bases of the pellet, and the movement of the pellet with respect to the laser beam is reduced. A dust removal method, which is a translational movement along the main axis of the pellet associated with a rotational movement of the pellet about its main axis. 6. A method according to claim 5, wherein several pellets are joined together at their bases to form a column exposed to the impact of the laser beam. 7. The method of claim 1, wherein the laser beam has a rectangular cross section. 8. The dust removing method according to claim 1, wherein the dust resulting from the removal of dust deposits is removed by suction. 9. The method of claim 8, wherein the surface exposed to the impact of the laser beam is cleaned by a gas flow in a direction that facilitates suction of products resulting from dust deposition. . 10. The method according to claim 1, wherein the dust removal is performed in a closed enclosure. 11. An apparatus for removing dust from at least one nuclear fuel pellet that has been ground so that dust of a material forming the nuclear fuel pellet remains on the surface of the pellet, wherein dust is removed from the surface. A dust removing apparatus, comprising: means for exposing the surface to impact by a laser beam having suitable characteristics; and means for removing a product resulting from the removal of the dust deposit from the surface. 12. The apparatus of claim 11, further comprising a mechanism for moving the pellet to expose the entire surface to a laser beam impact such that the laser beam is fixed and the entire surface is subjected to dust removal. Removal device. 13. The dust removing apparatus according to claim 12, wherein the mechanism for moving the pellets has two rotating rollers for supporting the pellets and transmitting the rotational motion to the pellets. 14. The dust removing apparatus according to claim 12, wherein the mechanism for moving the pellet has means for rotating the pellet and means for translating the pellet relative to the laser beam. 15. The dust removing apparatus according to claim 12, further comprising: means for uniting several pellets; and means for exposing the plurality of pellets to laser beam impact. 16. Apparatus according to claim 12, comprising means for allowing the laser beam to have a rectangular cross section. 17. The duct removing device according to claim 11, wherein the means for removing a product resulting from dust deposit removal has a suction device. 18. The apparatus according to claim 17, wherein the suction device has a member for collecting a product generated from dust deposit removal, and the member is a transparent portion that can transmit the laser beam. A transparent portion arranged to transmit the laser beam to the pellet via the interior of the member, and an opening to be disposed facing the pellet, wherein the passage of the laser beam and the generation A duct removal device having an opening for allowing suction of objects, at least one port for introducing a carrier gas in the direction of the pellets, and at least one port for aspirating a carrier gas carrying the product. 19. The apparatus according to claim 18, wherein the carrier gas inlet is disposed between the transparent portion and the opening near the transparent portion. 20. The apparatus of claim 19, wherein said opening is shaped to overlie a pellet. 21. The apparatus of claim 18, wherein the pellet motion mechanism comprises two rotating rollers for supporting and imparting rotational motion to the pellet, wherein the member comprises:
An opening in the member functions as a carrier gas inlet, and at least one carrier gas suction port is disposed near the opening, the assembly being located below an assembly formed by the rotating rollers and pellets. Has dust removal equipment. 22. The dust removing apparatus according to claim 21, wherein the transparent portion for transmitting the laser beam is disposed so as to displace the laser beam obliquely with respect to a vertical line. 23. The apparatus according to claim 21, further comprising a mirror for guiding the laser beam to the pellet, the mirror being swingable to guide the laser beam to the rotating roller to clean the rotating roller. A dust removal device.